<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../js/vue.js"></script>
</head>

<body>
    <div id="root">
        <h2>今天天气很{{info}}</h2>
        <button @click="changeWeather">切换天气</button>
    </div>

</body>
<!-- 
    监视属性watch
        1. 当被监视的属性变化时, 回调函数自动调用, 进行相关操作
        2. 监视的属性必须存在, 才能进行监视
        3. 监视的两种写法:
            (1). new Vue时传入watch配置
            (2). 通过vm.$watch监视

 -->

</html>
<script>
    const vm = new Vue({
        el: '#root',
        data: {
            isHost: true
        },
        computed: {
            info() {
                return this.isHost ? '炎热' : '凉爽'
            }
        },
        methods: {
            changeWeather() {
                this.isHost = !this.isHost
            }
        },
        // watch: {
        //     isHost: {
        //         immediate: true,
        //         handler(newValue, oldValue) {
        //             console.log('isHost被修改了', newValue, oldValue);
        //         }
        //     }
        // }

    })
    vm.$watch('isHost', {
        immediate: true,
        handler(newValue, oldValue) {
            console.log('isHost被修改了', newValue, oldValue);
        }
    })

</script>